1. Field Of the Invention
The present invention concerns a microwave amplifier working in both linear and logic modes, having matching circuits which are modified as compared with standard amplifiers, so that it can have a very wide pass band of between 1 and 100 Gigahertz. In this amplifier, the gates of the transistors of the amplifier stages are biased at stable voltages given by circuits comprising only transistors and diodes, to the exclusion of circuits having resistors, chokes and capacitors which are commonly used in the prior art. The amplifier according to the invention is of a combined linear and logic type, for its linear amplification part is biased by assemblies which are habitually used in logic applications.
The invention can be applied essentially to microwave circuits made with gallium arsenide or, more generally, with fast materials of the III-V group.
2. Description of the Prior Art
It is known that logic circuits made of GaAs can work at frequencies ranging from some Mhz up to several tens of Ghz. In a ring assembly, the logic inverters may have minimum propagation times of about ten picoseconds. This shows that the logic circuits have a very wide pass band, covering all the frequencies up to 100 Ghz. The reason why a logic circuit has a very wide pass band is that, for the logic levels to be compatible with one other, the load and bias elements are made of semiconducting components such as diodes and transistors, which themselves have a very wide pass band.
It is also known that, in linear circuits, a microwave transistor can achieve amplification up to very high frequencies, but the problems related to microwave amplifiers arise out of the matching circuits, namely the biasing and load circuits, which have fairly narrow pass bands. For, the biasing circuits, for example, are generally made with a bias voltage, by means of resistors, chokes and capacitors which themselves have fairly narrow pass bands.
The invention therefore consists in the application of known logic circuits to match and bias the amplifier stages so as to obtain a linear amplifier with a very wide pass band.
To work efficiently, a microwave transistor (very usually of the field effect type) must have its gate biased, at neutral point, at a bias voltage which is accurately chosen according to the graphs of the transistor's intensity/voltage characteristics, but this bias voltage should be stable. It is standard practice to provide the bias voltage from a source which delivers it through a resistor which is series-mounted with a choke, a capacitor being mounted in parallel and grounded. These components take up a great deal of space in monolithic integration. This means that they are not desired for the manufacture of microwave integrated circuits made of GaAs but, in addition, they have a narrow pass band which limits that of the amplifier.
According to the invention, the gate of at least one field effect transistor in a microwave amplifier is biased, at neutral point, by a known logic circuit comprising a BFL (Buffered FET Logic) type inverter, followed by a shifter. The input signal is applied simultaneously to the gate of the input field-effect transistor of the inverter and to the gate of the input field effect transistor of the amplifier, which is itself connected to the output of the phase shifter: whenever the bias voltage at the gate of the amplifier transistor varies, the reaction of the BFL gate and the shifter brings this voltage back to its initial value.
Furthermore, the amplifier has at least one stage in which two transistors are mounted as differential amplifiers, in the manner of a NOR logic gate. The gate of the first transistor receives the input signal. The gate of the second transistor is biased by the output signal of this stage, through a circuit comprising an inverter and a shifter which are identical to the above-described circuit: this assembly enables the stabilization of the bias voltage of the second transistor of this stage and, at the same time, of the bias voltage of the input transistor of a following amplification stage. Several amplification stages can then be cascade-mounted since their inputs and their outputs are self-matched.